Many materials such as adhesives must be heated in order to bring them to the proper viscosity for application to a substrate. For instance, in a hot-melt adhesive dispensing system, solid adhesive material is melted in a heated tank and distributed in liquid form to one or more remote dispensing guns through a duct in a manifold block. The manifold block is heated to keep the adhesive in a flowable, liquid state. To increase the rate of heat transfer from the manifold block to the adhesive, it has been proposed to press fit an insert of thermally conductive material into the duct. The press fit insures intimate mechanical and hence, thermal contact between the wall of the duct and the insert. The insert includes a plurality of longitudinal flutes disposed about its periphery which divide the duct into a series of passageways thereby increasing the heated surface area in contact with the adhesive.
Heat exchangers incorporating inserts for providing increased surface area are well known. For example, U.S. Pat. Nos. 2,726,681 and 2,731,709 to Gaddis et al. describe an internally finned heat exchanger tube and method of making the same whereby a plurality of channel members are temporarily secured at their base to a polygonal supporting rod. The assembly is then fitted into a tube and the supporting rod is removed. The channel members are bonded to the inner wall of the tube by copper brazing. Brazing secures member to the tube with little thermal resistance, but is undesirable from a production standpoint. Brazing is relatively slow, subject to high scrap rates and requires special equipment for heating and proper flux removal afterward. It is also difficult to braze inside a massive member such as a manifold block since considerable heat input is required. Unless performed in an inert atmosphere and depending on the material used, brazing can result in the formation of thermally insulating oxides and may necessitate subsequent heat treating to relieve stresses or restore metallurgical properties. Once brazed, the insert is permanent and cannot be easily removed. This is a serious drawback in applications where it may be necessary to remove the insert for cleaning or unclogging.
U.S. Pat. No. 2,895,508 to Drake shows an insert having a plurality of radially extending legs terminating in foot portions. The insert is force fitted into a tube to bring the feet into intimate mechanical contact with the inner wall of the tube. The insert may deform elastically for a resilient fit or the interference may be such that the feet and tube will cut into one another. A similar arrangement is shown in U.S. Pat. No, 3,871,407 to Bykov et al. which discloses forming the ribs of an insert as wedges having pointed ends which displace the wall of a tube into which the insert is press fitted thereby improving thermal conductivity.
Press fit techniques are troublesome because they require close tolerances. Too much interference can result in galling or cracking the insert or tube while too little interference produces a poor thermal joint. Even with proper tolerances, it is often difficult to apply sufficient force to press fit an insert of significant length. Another problem with press fitting is that, like brazing, the insert is permanently installed and cannot be removed without considerable difficulty. Further, press fits which displace metal can also weaken the tube to a degree which is not easy to predict or control. This can be a serious problem where high operating pressures are involved or where the heat exchanger must contain hazardous materials.
Accordingly, there exists a need for a heat exchanger having an insert which can be installed in a duct easily without requiring a large insertion force and without galling or otherwise compromising the structural integrity of the insert or the duct. Further, there is a need for such a heat exchanger having an insert which provides good thermal contact between the insert and the duct without brazing or welding. There further exists a need for a heat exchanger including an insert which, following installation in a duct, can be easily removed as required for maintenance.